As design of some compact air vehicles develops, especially those with blended wing bodies, the engine of the vehicle may be located at the rear end of an inlet duct which is of deviated or serpentine form. The complex geometry of the inlet duct may be convoluted in more than one dimension to achieve a required spatial footprint in an optimised design. When carrying out tests to assess engine characteristics within such an installation, it has been found by the applicant that there was an unexpected drop in performance at particular operating conditions.